1========= 2Bus Types 3========= 4 5Definition 6~~~~~~~~~~ 7See the kerneldoc for the struct bus_type. 8 9int bus_register(struct bus_type * bus); 10 11 12Declaration 13~~~~~~~~~~~ 14 15Each bus type in the kernel (PCI, USB, etc) should declare one static 16object of this type. They must initialize the name field, and may 17optionally initialize the match callback:: 18 19 struct bus_type pci_bus_type = { 20 .name = "pci", 21 .match = pci_bus_match, 22 }; 23 24The structure should be exported to drivers in a header file: 25 26extern struct bus_type pci_bus_type; 27 28 29Registration 30~~~~~~~~~~~~ 31 32When a bus driver is initialized, it calls bus_register. This 33initializes the rest of the fields in the bus object and inserts it 34into a global list of bus types. Once the bus object is registered, 35the fields in it are usable by the bus driver. 36 37 38Callbacks 39~~~~~~~~~ 40 41match(): Attaching Drivers to Devices 42~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 43 44The format of device ID structures and the semantics for comparing 45them are inherently bus-specific. Drivers typically declare an array 46of device IDs of devices they support that reside in a bus-specific 47driver structure. 48 49The purpose of the match callback is to give the bus an opportunity to 50determine if a particular driver supports a particular device by 51comparing the device IDs the driver supports with the device ID of a 52particular device, without sacrificing bus-specific functionality or 53type-safety. 54 55When a driver is registered with the bus, the bus's list of devices is 56iterated over, and the match callback is called for each device that 57does not have a driver associated with it. 58 59 60 61Device and Driver Lists 62~~~~~~~~~~~~~~~~~~~~~~~ 63 64The lists of devices and drivers are intended to replace the local 65lists that many buses keep. They are lists of struct devices and 66struct device_drivers, respectively. Bus drivers are free to use the 67lists as they please, but conversion to the bus-specific type may be 68necessary. 69 70The LDM core provides helper functions for iterating over each list:: 71 72 int bus_for_each_dev(struct bus_type * bus, struct device * start, 73 void * data, 74 int (*fn)(struct device *, void *)); 75 76 int bus_for_each_drv(struct bus_type * bus, struct device_driver * start, 77 void * data, int (*fn)(struct device_driver *, void *)); 78 79These helpers iterate over the respective list, and call the callback 80for each device or driver in the list. All list accesses are 81synchronized by taking the bus's lock (read currently). The reference 82count on each object in the list is incremented before the callback is 83called; it is decremented after the next object has been obtained. The 84lock is not held when calling the callback. 85 86 87sysfs 88~~~~~~~~ 89There is a top-level directory named 'bus'. 90 91Each bus gets a directory in the bus directory, along with two default 92directories:: 93 94 /sys/bus/pci/ 95 |-- devices 96 `-- drivers 97 98Drivers registered with the bus get a directory in the bus's drivers 99directory:: 100 101 /sys/bus/pci/ 102 |-- devices 103 `-- drivers 104 |-- Intel ICH 105 |-- Intel ICH Joystick 106 |-- agpgart 107 `-- e100 108 109Each device that is discovered on a bus of that type gets a symlink in 110the bus's devices directory to the device's directory in the physical 111hierarchy:: 112 113 /sys/bus/pci/ 114 |-- devices 115 | |-- 00:00.0 -> ../../../root/pci0/00:00.0 116 | |-- 00:01.0 -> ../../../root/pci0/00:01.0 117 | `-- 00:02.0 -> ../../../root/pci0/00:02.0 118 `-- drivers 119 120 121Exporting Attributes 122~~~~~~~~~~~~~~~~~~~~ 123 124:: 125 126 struct bus_attribute { 127 struct attribute attr; 128 ssize_t (*show)(const struct bus_type *, char * buf); 129 ssize_t (*store)(const struct bus_type *, const char * buf, size_t count); 130 }; 131 132Bus drivers can export attributes using the BUS_ATTR_RW macro that works 133similarly to the DEVICE_ATTR_RW macro for devices. For example, a 134definition like this:: 135 136 static BUS_ATTR_RW(debug); 137 138is equivalent to declaring:: 139 140 static bus_attribute bus_attr_debug; 141 142This can then be used to add and remove the attribute from the bus's 143sysfs directory using:: 144 145 int bus_create_file(struct bus_type *, struct bus_attribute *); 146 void bus_remove_file(struct bus_type *, struct bus_attribute *); 147